1. Field of the Invention
The present invention relates to a base station and a communication system which perform transmission timing control in packet access. Specifically, the present invention relates to a base station and a communication system which orthogonalize uplink signals from multiple mobile stations by controlling transmission timings of signals in packet access to be synchronized among mobile stations which are situated in multiple sectors in the same base station.
2. Description of the Related Art
In a cellular system with a multiple-cell environment, it is proposed that an increase in the system capacity be achieved by employing one-cell frequency reuse that uses the same frequency among adjacent cells. In order to achieve one-cell frequency reuse, gains for reducing interference from the adjacent cells are needed. Typically, one-cell frequency reuse is achieved by reducing interference from the adjacent cells by means of a spreading gain obtained from spreading and a channel coding gain obtained from error correcting coding according to the principles of CDMA (Code Division Multiple Access). In addition to one-cell frequency reuse, a further increase in the system capacity is achieved by dividing one cell into multiple areas called sectors and performing communications independently in each sector based on the principles of CDMA.
When one-cell frequency reuse is achieved, each mobile station can simultaneously access the base station in uplink communications from the mobile station to the base station. The reason why each mobile station can simultaneously access the base station is that the base station can distinguish and demodulate (decode) signals transmitted from multiple mobile stations by means of the spreading gain, even if a collision occurs among signals transmitted from the multiple mobile stations. However, the collision indeed occurs among signals transmitted from the multiple mobile stations when the mobile stations simultaneously access the base station. Although the effect of this collision can be reduced to some extent by means of the spreading gain, the signals interfere with each other (multiple-access interference), which results in degrading communication quality. The multiple-access interference occurs not only within a single cell but also among sectors. In other words, the mobile station close to a sector boundary can experience multiple-access interference in both sectors.
In order to reduce multiple-access interference, it is proposed that transmission timing control be performed in CDMA communications employing one-cell frequency reuse (for example, Non-Patent References 1 and 2).
With reference to FIG. 1, the transmission timing control is described. FIG. 1 shows a schematic flowchart illustrating transmission timing control in accordance with the related art. First, the base station BS transmits a pilot signal on a common pilot channel to two mobile stations MS1 and MS2 in the same sector (S1). The mobile stations MS1 and MS2 which receive the pilot signal on the common pilot channel start transmitting signals based on the reception timing of the pilot signal (S2). Specifically, because the distance from the base station varies depending on the mobile station, the mobile station MS2 close to the base station receives the pilot signal earlier than the mobile station MS1 far from the mobile station. The mobile stations MS1 and MS2 respectively transmit signals to the base station BS according to the arrival time of the pilot signal on the common pilot channel. The base station BS measures a reception timing difference between these signals (S3). The base station BS generates transmission timing control information based on the reception timing difference and transmits it to the mobile stations MS1 and MS2 (S4). Each mobile station starts communications based on the received transmission timing control information (S5). When each mobile station controls the transmission timing in this manner, the reception timings of the signals from the mobile stations MS1 and MS2 are synchronized at the base station BS (S6).
The transmission timing control allows reception timings of paths with maximum reception power for the respective mobile stations to be synchronized at the base station, and allows signals from the mobile stations to be orthogonal on the synchronized paths with maximum reception power. Accordingly, the transmission timing control can reduce multiple-access interference and improve communication quality.
[Non-Patent Reference 1] E. Hong, S. Hwang, K. Kim, and K. Whang, “Synchronous transmission technique for the reverse link in DS-CDMA,” IEEE Trans. on Commun., vol. 47, no. 11, pp. 1632-1635, November 1999
[Non-Patent Reference 2] “Study report for uplink synchronous transmission scheme,” 3GPP, 3G TR 25.854